Wide band technology is increasingly being developed for communications and other applications. Unlike narrow band systems, which operate at specific frequencies, wide band systems can transmit and receive sequences of very short pulses, i.e. pulses generated from a broad range or bandwidth of frequencies (typically several MHz to several GHz) of the electromagnetic spectrum. The input to a wide band antenna is typically from one or more pulsed sources, and the antenna is required to radiate incident energy into free space.
Clearly, optimising performance is a key consideration in antenna design. Regardless of the type and configuration of an antenna, its performance can be characterised by (at least) the following metrics:
i) Impedance bandwidth
ii) Directive Gain
iii) Efficiency
Antenna impedance, and the radio frequencies over which that impedance is maintained, are critical. It is essential that the antenna present an acceptable impedance match over the frequency band(s) of operation. Antenna impedance and the quality of the impedance match are most commonly characterized by either return loss (represented by the scattering parameter S11) or Voltage Standing Wave Ratio (VSWR)—these two parameters are simply different formats of exactly the same impedance data. S11 or return loss, then, is a measure of how much power is reflected back at the antenna port due to mismatch from the transmission line.
Bandwidth refers to the range of frequencies a given return loss can be maintained. Since return loss is a measurement of how much power the antenna accepts from the transmission line, the impedance of the antenna must match the impedance of the transmission line for maximum power transfer. However, the impedance of the antenna changes with frequency, resulting in a limited range (or ranges) that the antenna can be matched to the transmission line.
In general terms, gain is a key performance figure that combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction. The gain of an antenna will vary across its operating bandwidth, usually peaking at the or each resonant frequency.
Antenna efficiency is a measure of what portion of the power supplied to the antenna, including any reflection loss, is actually radiated by the antenna and it is well known in the art that, in order to maximise transmission efficiency, the impedance of the source can be matched, via the antenna, to that of the medium in which the signals are to be transmitted. The medium in which signals are to be transmitted is often free space.
Horn antennas have been used for many years as a means of matching the impedance of a transmission line to that of free space and directing the radiated energy in a controlled manner by virtue of their gain characteristics. The horn antenna can be considered as an RF transformer or impedance match between the waveguide feed (supplying the input signal) and free space which has an impedance of 377 Ohms.
An accepted method of broadening the range of frequencies over which a horn antenna is impedance-matched is to introduce ridges within the horn. These are often combined with a dielectric lens or tapered periodic surface in order to aid in limiting diffraction from the horn edges, thus helping to limit the beamwidth at low frequencies. The use of ridges essentially extends the upper frequency limit over which the antenna remains well matched, since this is a function of the aperture dimensions.
A horn antenna of the types described above could be designed which permits a significant proportion of the incident energy to be radiated over a broad band. However, for the proposed application, which may involve several high-power input sources, for example, several signal generators such as microwave frequency oscillators (MFOs), the inputs may first need to be combined before being fed to the single horn antenna. This is not generally considered to be feasible at high powers, principally due to the high risk of dielectric breakdown at the combined high power, and losses in the combination process. To overcome this problem, the available antenna aperture can instead be sub-divided into a number of smaller regions, with sources attached to each region.
Alternative antenna designs comprise arrays of elements where the radiation from a number of such elements can be coherently summed in a particular direction to form a main beam. The aim in such an antenna design is to generate a single lobe from the antenna array, substantially uncorrupted by so-called grating lobes, which are spurious lobes resulting from standing waves in the elements. To minimise such grating lobe corruption, it is common for such arrays to be constructed so as to maximise the element spacing (thereby using a minimum number of elements whilst maintaining a sufficient impedance match for a specified area or aperture, to avoid the onset of grating lobes at particular scan angles. Such a spacing of elements tends to decrease efficiency due to compromised impedance matching.
Travelling wave antenna elements have been proposed for such antenna designs, for example, by Godard et al, “Size reduction and radiation optimization on UWB antenna”, RADAR CONFERENCE, IEEE 2008. In this document, an antenna element is described having upper and lower conductive loop, the upper conductive loop comprising an upper conductor and a first conductive blade that tapers outwardly to form a flare portion adjacent a distal end of the upper conductor, the lower conductive loop comprising a base conductor and a second conductive blade that tapers outwardly to form a flare portion adjacent a distal end of the base conductor, the conductive loops being arranged and configured such that the outer edges of the first and second conductive blade members face each other to define a notch that tapers outwardly from the feed region of the antenna element. A conductive vane is provided between the upper conductor and the first conductive blade member to define two loops within the upper conductive loop. However, the antenna documented in this paper is designed to have one set of predefined characteristics for use in a very specific application, and the configuration of the antenna element (and the associated characteristics) are met, to a large extent, by experimentation. The field of travelling wave antennas has, thus far, received relatively very little attention compared with other types of antenna and, as such, although this and other academic papers exist that document specific travelling wave antenna designs, they provide little more general design principles for this type of antenna element that could be applied to a method of manufacturing such elements having differing characteristics and for different respective applications.
Thus, aspects of the present invention seek to provide a method of manufacturing a travelling wave antenna element that can be adapted to the manufacture of such elements having different respective performance characteristics to meet different respective needs.
More generally, aspects of the present invention seek to provide an efficient wide band antenna that radiates energy, possibly input from at least one high power pulsed source and fed via a co-axial line, into free space, which can be designed to have a predetermined cut-off frequency and its performance optimised over a specified frequency band of operation.